The history of the stereoscopic cinema has attributes in common with the addition of other reality heightening technology for the projection of motion pictures. This includes the addition of color, sound, and wide aspect ratio projection. In every case the initial technology called for the use of multiple projectors or playback devices to achieve the desired enhancement. For example, in the late '20's, for the original Vitaphone sound system developed by Western Electric, a phonograph turntable was synchronized to the motion picture projector to provide lip synchronized sound. In the case of the wide screen Cinerama process in the early 1950's, three interlocked projectors were used to achieve the desired aspect ratio by means of a triptych on a curved screen. Cinerama was superseded by techniques using anamorphic lenses for 35 mm projection, or by means of the projection of wide aspect ratio 70 mm. Both of these approaches are superior in that they use a single projector to achieve the desired effect.
For color motion pictures, in the silent era, at least on a prototype basis, multiple projectors were used to produce additive color images, but soon techniques were evolved to do away with multiple projectors. Subframes were incorporated into the existing motion picture frame, with each subframe carrying a record of a portion of the visible spectrum. When projected through appropriate color filters and lenses, the resultant image was an additive color display.
However, such displays proved to be unsatisfactory and gave way to integral print stock incorporating the color information in three layers for the projection of subtractive color images.
For the projection of stereoscopic motion picture films, the ideal solution, at least from a technological, if not from a product point of view, is the Vectograph, which was the subject of considerable effort on the part of Mahler and Land, of the Polaroid Corporation. The Vectograph encoded the polarization characteristics of the image on the frame of a single piece of film, as described in U.S. Pat. No. 2,289,714, issued Jul. 14, 1942. No special projection lens or polarizing filter is required. The viewer wears the standard polarizing glasses. Footage recently viewed of a Vectograph test made by Polaroid in the 1950's showed that the process produced a beautiful result. The Vectograph process is an imbibition process, like that of the Technicolor technique.
The history of stereoscopic projection of motion pictures, to some extent, recapitulates the other technological innovations which have been briefly described above. Unfortunately, the ultimate technical solution, the incorporation of both left and right perspective viewpoints in an integral form, as would have been possible with the Vectograph, is no longer viable with the abandonment, in the West, of the Technicolor imbibition process. The commercial possibility of Vectograph release prints has vanished.
Initially, when stereoscopic films were projected in the late 30's, for exhibition at world's fairs, mechanically or electrically interlocked projectors were used. In the early 1950's the same approach was tried in neighborhood theaters, using the two projectors in the booth required for changeovers. The technical challenges involved in this operation; the frame for frame synchronization of the prints, the control of the phase relationship of the projector shutters, and the coordination of the optical characteristics of the two images, became a quality control problem which was never solved for neighborhood theaters. Today, it would be difficult to use such a technique, because most theaters have only one projector with a large reel platter, eliminating the need for changeovers between reels.
The practice of projecting stereoscopic motion pictures using two projectors continues to this day for world's fair exhibition. In venues of this type, by using trained and motivated technicians, it is possible to use interlocked projectors. This technique is in daily use by Disney and other organizations.
Commercial attempts were made beginning in the early 1950's to incorporate two subframes, one for the left perspective and the other for the right perspective on a single piece of film. Various types of proposals were made which can be found in Lipton's Foundations of the Stereoscopic Cinema, Van Nostrand and Reinhold (1982). Eventually, a technique was accepted in which two subframes are incorporated into the area of an existing 35 mm motion picture frame, reminiscent of the approach used for additive color motion pictures. These subframes are arranged as shown in FIG. 1. The result is a format which has an aspect ratio which is wider than that used for so-called "scope" films. Generally speaking, the aspect ratio of motion pictures these days, when projected in theatrical cinemas, is (in the United States) between 1.85:1 (wide-screen) and 2.35:1 (scope). The latter aspect ratio is usually achieved using anamorphic projection lenses. A typical practice is to project all films on a screen with masking fixed at a 2:1 aspect ratio. Therefore, the aspect ratio of the subframe stereoscopic format, at approximately 2.5:1, provides a wider projected image than is desirable, or is in common practice. As the inventor has recognized, the wider than necessary aspect ratio makes it possible to mask a portion of the subframe area to allow for the location of an index mark.
In order to properly display a motion picture using the above-and-below (also known as the over-and-under) format described here and illustrated in FIG. 1, the projection device must accomplish the tasks of simultaneously superimposing the two images on the projection screen and of also polarization encoding the images so that polarizing selection devices may be used by audience members. Various types of projection devices have been described in the literature and manufactured and offered for sale. There projection devices use lenses, mirrors, or prisms, or combinations of such elements in their construction. They are either used in place of or as an add-on for use in conjunction with the existing projection lens. A partial list of such devices includes Kent et al., U.S. Pat. No. 3,851,955, Kent et al., U.S. Pat. No. 4,017,166, Kent et al., U.S. Pat. No. 4,183,633, Condon, U.S. Pat. No. 4,235,503, and Marks et al., U.S. Pat. No. 4,372,656.
The projection optics employ sheet polarizers to polarize the image of each subframe. These polarizers are usually of the linear type, and their axes are orthogonal and at 45 degrees to the horizontal, following the industry practice. As mentioned previously, the means for combining or superimposing the two images can involve the use of prisms, lenses, or mirrors. Many such devices were sold to exhibitors in the early 1980's, when there was a brief revival of the three-dimensional cinema.
Different approaches have been used for producing the above-and-below subframe format. It is known to those who are familiar with the art that it is possible to use interlocked dual cameras to produce stereoscopic photography and then to optically print from the resultant films the above-and-below format shown in FIG. 1. Some inventors have decided that it would be desirable to produce this format at the time of photography, thereby simplifying the process since only one camera is needed and optical printing is eliminated. These inventors and inventions include Bernier, U.S. Pat. No. 3,531,191, Hoch, U.S. Pat. No. 3,825,328, Marks et al., U.S. Pat. No. 3,990,087, Marks et al., U.S. Pat. No. 4,175,829, Bukowski, U.S. Pat. No. 4,436,369, and Condon, U.S. Pat. No. 4,464,028. By no means is this list complete, but it is representative and shows that there has been, as is the case for projection optics, considerable effort expended in this art. Many of these patents have been the basis for products which have been or are now commercially employed in the theatrical film industry.
Tables 1 and 2, set forth in FIG. 1(a) and FIG. 1(b) respectively, provide the dimensions for two conventional variants of the above-and-below film format shown in FIG. 1. FIGS. 1, 1(a), and 1(b) are provided from the Society of Motion Picture and Television Engineers working group formed to establish standards for the projection of 35 mm theatrical stereoscopic motion pictures. One variant (represented by FIG. 1(a)) is called the symmetrical format and the other (represented by FIG. 1(b)) the asymmetrical format. In the case of the symmetrical format, the center-to-center distance between any two successive subframes (i.e., subframe A and subframe B shown in FIG. 1 and the subframes adjacent to them) is identical, namely 0.374 inches. In the case of the asymmetrical format, the subframe center-to-center distance will vary depending upon whether one is measuring the center-to-center distance between an A subframe and the B subframe above it, or the center-to-center distance between a B subframe and the A subframe above it. This difference between the symmetrical format and the asymmetrical format is an important concept because the invention disclosed here can only be made to work with the symmetrical subframe format, for example that given in Table 1. If the invention disclosed herein were to be applied to film using the asymmetrical subframe arrangement shown in Table 2, or for any other format which is not symmetrical, the resultant projected image would exhibit vertical parallax. Vertical parallax, as is well known in the art, and explained in Lipton's Foundations of the Stereoscopic Cinema, cited above, is undesirable when present in stereoscopic projection.
Amongst the many challenges that exist with regard to using the subframe technique for stereoscopic motion pictures, two of the most critical are projecting images which have symmetrical illumination and projecting images which remain stereoscopic rather than pseudostereoscopic. The first concern has been addressed in Lipton's U.S. Pat. No. 4,472,037. Using the technique described in U.S. Pat. No. 4,472,037, it is possible rapidly to observe the relative intensity of the two projected subframes to control the illumination so that the vignetting can be made congruent for both left and right projected image fields. There must be a point-for-point correspondence in terms of illumination between the left and right subframes. If this condition cannot be achieved to within a given tolerance, the result for the viewer is substantial fatigue, or what people often call eyestrain. Even though the viewer may not be able to articulate the problem, it is one that needs to be addressed.
There is another problem which is of epidemic proportions in the projection of stereoscopic motion pictures using the subframe technique, and the invention disclosed herein addresses this problem. The particular subframe must remain associated with the appropriate polarizer. The individual light path of each subframe passes through a polarizing filter, which is part of the projection lens or attachment. It is necessary to maintain the proper location of each left and right subframe with the optical system. That is to say, the subframes must always be positioned properly within the projector aperture. They must always be positioned as shown in FIG. 1 so that subframe A is the right image and B the left image.
This is not always accomplished in practice, and there are two causes for the problem. One occurs in threading up the print in the projector. The second problem occurs when the reels are assembled. The sequence of subframes must remain intact. A loss of sequence can come about during assembly of the print for projection. During assembly the individual reels of the film are placed on a large platter, and at this time it is possible to lose the sequence. If the person assembling the print makes the splice at the subframe frameline, rather than the frame line, the proper sequence will be lost, and the projected image will become pseudoscopic (pseudostereoscopic). In this case the subframes, rather than having the proper sequence of "left, right, left, right, and so on," would follow the sequence "right, left, left, right, left, right," or the sequence "left, right, right, left, right, left." When viewing a projected pseudoscopic image, the left eye will see the image intended for the right eye and vice versa. Often the conflict of stereoscopic and other depth cues, such as perspective or interposition, will preclude the viewer from seeing that the image has become "inside out", but it will no longer be stereoscopic and the result will be disturbing for most viewers. The left eye will be seeing the right eye's image and vice versa, because each left subframe image is being polarized with the polarizing filter meant for the right subframes. For the case of selection with linearly polarized light, the new polarization axis will be orthogonal to its intended axis.
Since every person in the theater is wearing polarizing spectacles using linear polarizers whose axes are orthogonal, the loss of subframe sequence will consequently produce the pseudoscopic effect since the combination of polarizer at the projector lens or attachment and the analyzer in the eyewear will select the left image for the right eye and vice versa. One peculiar method to correct for this mistake is for the audience members to remove their 3D glasses and turn them upside down.
Stereoscopic motion pictures using the subframe technique will often have the print threaded improperly or have had an interrupted sequence. In the first case, the left subframe occupies the position intended for the right subframe, the entire film will be projected pseudoscopically. In the later case it is observed that the sequence will switch between stereoscopic and pseudoscopic projection at the reel changes where assembly took place. Apparently it is beyond the ability of the person assembling the reels to distinguish between the frame line and the sub-frame line, despite the fact that instructions are given with the print and despite the fact that various schemes have been employed to index the sub-frames to preclude the production of pseudoscopic images.
It is therefore the intention of the present invention to prevent the image from becoming pseudoscopic and to maintain the relationship of each subframe with its polarizer.
An additional goal of the present invention will come as no surprise to those who have seen stereoscopic motion pictures. When tipping the head only a few degrees, the result will be a ghost image, because of crosstalk between the left and right projected channels. In a stereoscopic system, it is necessary to have good isolation between the two image channels so that the right eye only sees its appropriate image and the left eye sees its appropriate image. Using linear polarizers requires the audience members to place their heads rigidly in one position. It is known to those versed in the art that this is a result of the Law of Malus, which tells us that even a small rotation of one of a pair of crossed polarizers away from the orthogonal axes condition will produce substantial transmission. Therefore, the extinction of the unwanted image cannot be guaranteed, unless the audience members' heads are held in a fixed position.
However, the use of circularly polarized light, as described by Land in U.S. Pat. No. 2,099,694, provides an adequate solution to the problem and tends to preclude crosstalk even as audience members tip their heads while seeking different and more comfortable head positions. However, commercially available circularly polarizing filters, of a type including sheet quarter wave retarder and sheet linear polarizer, have not had the same good extinction ratio of their linearly polarized counterparts. Hence, a further improvement of the present invention is to produce circularly polarized light which is of superior quality to that which may be produced by commercially available circular polarizers built of plastic sheet quarter wave retarders and sheet polarizers.